1. Field of the Invention
The present invention relates to a technique of detecting improper wiring in a parallel inverter system that drives a load such as a motor with a plurality of inverters connected in parallel.
2. Background Art
Cables for supplying electric power from an inverter to a motor are connected to the inverter and the motor through a connection unit such as a terminal block. This, in the work of laying the cables, sometimes causes improper cable connection to the terminal block, improper cable connection to the motor with an incorrect phase sequence or an erroneous omission of cable connection (hereinafter generically referred to as improper wiring). The driving of a motor without correcting such improper wiring results in an excessive current flowing in the motor, for example, to cause breakdowns of devices or a failure of the system. Hence, an operator must carry out checking beforehand so that there is no such improper wiring.
Here, as a system of related art detecting an unfinished connection between an inverter and a motor as a kind of improper wiring, the system shown in FIG. 4 and FIG. 5 is known, for example.
The system of the related art is that described in JP-A-7-20190 (paragraphs [0007] and [0008] and FIG. 1 and FIG. 2 etc.). FIG. 4 is a circuit diagram showing a motor driving system by an inverter in the related art. The motor driving system shown in FIG. 4 includes a rectifier circuit 101 converting a three-phase AC voltage to a DC voltage, a smoothing capacitor 102, an inverter section 103 including semiconductor switching devices Q1 to Q6, an AC motor M to which a three-phase AC voltage is supplied from the inverter section 103, current sensors 104 detecting their respective output currents in at least two phases of the inverter section 103, a current detector 105, a CPU (Central Processing Unit) 106 carrying out an operation of an output voltage instruction on the basis of the values of the detected currents to output the output voltage instruction, and a base driving circuit 107 producing driving signals for the switching devices Q1 to Q6 according to the output voltage instruction to output the driving signals.
FIG. 5 is a flow chart showing the operation of detecting a state of an unfinished connection between the inverter and the motor M in the motor driving system in the related art shown in FIG. 4.
First, by operating the rectifier circuit 101, the smoothing capacitor 102 is charged to bring the voltage across the smoothing capacitor 102 as the main circuit voltage of the inverter to the specified voltage (S101). In the next, by turning-on a switching device in each of two phases of the three phases in the inverter section 103, a closed circuit ranging from the DC circuit of the inverter to the motor M is formed. For example, by turning-on the switching devices Q1 and Q4, a closed circuit is formed for ascertaining whether or not a current flows from the U-phase to the V-phase in the inverter section 103 or, by turning-on the switching devices Q3 and Q2, a closed circuit is formed for ascertaining whether or not a current flows from the V-phase to the U-phase in the inverter section 103 (S102).
Similar operations are carried out with respect to all of the combinations of the two rest phases (S103 and S104). When it is ascertained that currents flow with respect to all of the combinations of two phases, a decision is made that the motor is normally connected to the output side of each of the phases of the inverter and the processing is made completed (“Yes” at S105). Moreover, when it is ascertained that no current flows with respect to any one of the combinations of two phases, a decision is made that the connection at the connection point in the corresponding phase is unfinished (“No” at S105, and S106).
Furthermore, in Japanese Patent No. 2,797,882 (paragraphs [0018] to [0021] and FIG. 1 and FIG. 2 etc.), a control system of a servomotor is described which the system detects a current at turning-on of switching devices in two-phases of an inverter to make a decision as to whether the servomotor is connected to the inverter or not.
In addition, in JP-A-2010-213557 (paragraphs [0007] to [0012] and FIG. 1 and FIG. 2 etc.), a control system of a three-phase synchronous motor is described. In the control system, output currents in at least two phases of an inverter driving the three-phase synchronous motor are detected to be subjected to orthogonal biaxial transformation, by which a q-axis current feedback signal, a fed back speed signal and a q-axis current instruction are obtained so that the improper wiring in two phases or in three phases of the motor is detected on the basis of thus obtained signals and instruction.
Patent Document 1: JP-A-7-20190 (paragraphs [0007] and [0008] and FIG. 1 and FIG. 2 etc.)
Patent Document 2: Japanese Patent No. 2,797,882 (paragraphs [0018] to [0021] and FIG. 1 and FIG. 2 etc.)
Patent Document 3: JP-A-2010-213557 (paragraphs [0007] to [0012] and FIG. 1 and FIG. 2 etc.)
Any of the systems of related art described in JP-A-7-20190, Japanese Patent No. 2,797,882 and JP-A-2010-213557 is a system for the case of driving a motor with a single inverter.
Compared with this, in a parallel inverter system driving a single motor with a plurality of inverters connected in parallel, cables are present by the number of the inverters connected in parallel even in the same phase. Thus, there is a problem in that it is unknown which inverter has improper wiring to the motor. In particular, in a parallel inverter system, when inductance components among inverters are small, the inverters brought into operation with improper wiring being included therein may cause a short circuit by some way of energization to result in an excessive current flowing in switching devices in the inverters, which might damage the system.
In addition, there is also a problem in that although in the system described in Japanese Patent No. 2,797,882, detection of an incorrect phase sequence in the wiring connected to the motor is possible (paragraph [0030] etc.), in each of the systems described in JP-A-7-20190 and JP-A-2010-213557, detection of an incorrect phase sequence is impossible.
Accordingly, it is an object of the invention to provide an improper wiring detecting system which is capable of surely detecting improper wiring such as an unfinished connection to a terminal block and incorrect connection in a parallel inverter system including a plurality of inverters. Moreover, it is another object of the invention to provide an improper wiring detecting system which makes detection of incorrect phase sequences possible as required.